This invention relates generally to semiconductor devices and more particularly to bipolar transistors.
As is known in the art, there exist a trend towards miniaturization of microwave and millimeter wave functions by use of monolithic microwave and millimeter wave integrated circuit technologies. It is also known that very high electron mobilities in Group III-V materials such as gallium arsenide make such materials preferred materials over more conventional semiconductor materials such as silicon for devices having a very high frequency of operation.
Most work in monolithic microwave integrated circuit technology involving gallium arsenide had centered around the field effect transistor, in particular the metal electrode semiconductor field effect transistor (MESFET), the high electron mobility transistor (HEMT), and the pseudomorphic transistor (PHEMT). In each of these devices, the principle feature of operation is that current in the device is carried parallel to the semiconductor surface. These transistors include ohmic source and drain electrodes which are disposed to make contact to a channel region. The electrical conductivity of the channel region is controlled by a gate electrode having a fairly short gate length, typically between 0.25 microns for very high frequency devices up to about 1 micron for lower frequency devices. In operation of such transistors, the source-drain current (i.e. channel current) has an upper limit which is related to the channel thickness. The maximum channel thickness, however, is determined by the maximum thickness which can be controlled by a gate electrode having a specified length. Further, to carry high channel current, the channel doping concentration should be relatively high. A high channel doping concentration, however, reduces the gate to source breakdown voltage. Since high breakdown voltages and high channel current are both desired for high output power, the relationship between channel doping and breakdown voltage electronically limits output power capabilities. Thus, output power from such millimeter wave and microwave FETS is limited significantly by electronic considerations rather than thermal considerations. Thus only a slight improvement in power can be obtained by using such devices in the pulse mode of operation.
One device which has been suggested as an alternative to the field effect transistor described above in particular for high power applications is the heterojunction bipolar transistor (HBT). The HBT as also the conventional bipolar transistor includes collector, base, and emitter layers disposed to form a pair of junctions. In general, a bipolar transistor is a three terminal device in which the upper layers (i.e. the base and emitter layers) are etched away in order to expose the underlying collector layer. Contacts are made to each of the layers to provide the three terminal device having a collector, emitter, and base contacts. Generally, for an NPN type of device where the P material is the base layer, a hole current is injected into the base which produces in response an electron current across the emitter-base junction. If the hole current can be made relatively small in comparison to the emitter current which is produced across the emitter-base junction in response to the hole or base current, then the relatively small hole current can control a relatively large emitter current and the difference between the amount of hole current and the amount of emitter current produced will provide amplification.
In silicon device technology, p-type and n-type dopants having relatively similar and relatively high hole and electron mobilities respectively are available which has permitted the development of a practical bipolar transistor with the use of alternating conductivity-type doped silicon layers. P-type doped gallium arsenide layers, however, have significantly lower hole mobilities than the electron mobilities of N-type doped GaAs. This problem has prevented practical development of a bipolar gallium arsenide transistor.
To overcome this problem, the heterojunction bipolar transistor (HBT) was conceived. The heterojunction bipolar transistor differs from the standard bipolar transistor in that the HBT incorporates an emitter material having a wider band gap energy than the band gap energy of material used in the base. This arrangement provides an abrupt energy discontinuity at the base-emitter junction. This discontinuity acts as a barrier to hole current which permits substantially higher p-type doping concentration in the base. Higher p-type doping compensates for the low hole mobility of p-type material in gallium arsenide thus making HBT devices practical.
The conventional III-V compound semiconductor HBT includes a semi-insulating gallium arsenide substrate having disposed thereover n-type doped gallium arsenide to form the collector layer, and p-type doped gallium arsenide to form the base layer. These layers form a first PN junction i.e. base-collector junction. Disposed over the base layer is an emitter layer of a wide band gap material such as aluminum gallium arsenide having a suitable compositional ratio of aluminum to gallium, typically of approximately 30% aluminum. This layer in combination with the base layer provides a base-emitter heterojunction.
Several problems exist with this structure, commonly referred to as the "collector down" structure. The first set of problems involves the difficulties in fabricating such a device. Since most chemical etchants dissolve gallium arsenide, it is difficult to find a selective etchant which will etch the emitter layer and stop at the base layer to allow contact formation to the base. Moreover, if a selective chemical etchant were found which etched the AlGaAs but not the GaAs it is likely that the selective chemical etchant would nevertheless provide non-uniform surfaces over the base which would make good ohmic contact to the base layer more difficult resulting in reduced device yield. Moreover, providing good ohmic contact with low contact resistance to p-type GaAs is also relatively difficult.
With the collector/down structure, it is also difficult to ground the emitter. In general, many applications for HBTS call for a grounded emitter. A typical "collector down" HBT would include a plurality of cells including emitter-base-collector regions disposed laterally across a composite transistor structure. The collector, base, and emitter cells would be electrically contacted by spaced conductive fingers disposed on said cells which would be connected together at respective common collector, base, and emitter electrodes. It is relatively easy to interconnect the base fingers and the collector fingers on the top surface of the semiconductor substrate. However, in order to connect the emitter fingers, it is required to form an airbridge overlay across the collector and base regions. An airbridge overlay is generally undesirable. The airbridge overlay provides associated parasitic resistance and inductance, as well as, fabrication difficulties during manufacturing of the transistor. Moreover, in the grounded emitter configuration, the more commonly used configuration, it is required to ground the emitter from the upper surface of the device. This also presents a problem at microwave and millimeter wave frequencies. Further, since the HBT is an inherently thermally limited device rather than an electronic limited device as is the field effect transistor, it would be highly desirable to provide good thermal heatsinking of the emitter base junction. The arrangement described above, however, does not provide such a capability, since gallium arsenide has relatively poor thermal conductivity, and the emitter-base junction is separated from the heatsink on the GaAs substrate by the collector-base junction. A final problem with the devices described above involves the parasitic capacitance between the base and the collector. Since the base is defined by etching through the emitter to expose the underlying base layer, there exists a substantial portion of the base layer which is in contact with the collector layer but is not in contact with the emitter layer. This provides a depletion region at the junction which has to be charged and discharged during operation of HBT resulting in degraded performance at high operating frequencies.
Several solutions have been proposed in the art to address individually some of the problems mentioned above. One proposed solution is the so-called "emitter down" HBT or "collector up" HBT, as proposed by Kroemer, Hetero-Structure Bipolar Transistors and Integrated Circuits, Proceedings of IEEE, Vol. 70, No. 1, January 1982. Kroemer describes the device as a "collector up" structure in which the emitter is disposed on the substrate and has a larger area than the collector. The collector is diffused into a relatively wide base layer and is provided planar with the base. Since a base, collector P-N junction is formed completely around the collector, only such areas of the emitter directly underlying the collector contribute to charge injection. Kroemer indicates that certain device electronic advantages such as reduced collector base capacitance result with such a structure, since the area of the collector is reduced by having the collector as a top layer which is diffused through the underlying base region.
A second type of HBT transistor is described in an article entitled AlGaAs/InGaAs/GaAs Strained Layer HBT, Heterojunction Bipolar Transistor by Sullivan et al., Electronics Letters, Apr. 10, 1986, Vol. 22, No. 8, pp. 419-421. In this paper, the authors describe the use of a low band gap material such as indium gallium arsenide for the base material in an emitter up structure. The motivation for use of the indium gallium arsenide in this arrangement is to provide an effective increase in hole mobility in the base. A potential drawback with this approach is that the Al content of the AlGaAs emitter is reduced. This results because InGaAs has a narrower band gap than GaAs. The authors speculate as to whether or not improved performance may be obtained from such an arrangement.
Nevertheless, it would be desirable to provide a HBT structure which takes advantage of the thermally limited nature of a bipolar transistor, which is relatively easy to fabricate, which facilitates fabrication of low resistivity ohmic base, collector and emitter contacts, has reduced base parasitic capacitance and inherently has lower thermal resistance between the base-emitter junction and heatsink.